Carbonylation of methanol produces acetic acid. Prior to 1970, acetic acid was made using cobalt catalysts. A rhodium carbonyl iodide catalyst was developed in 1970 by Monsanto. The rhodium catalyst is considerably more active than the cobalt catalyst. Most importantly, the rhodium catalyst gives high selectivity to acetic acid.
One problem associated with the original Monsanto process is that a large amount of water (about 14%) is needed to produce hydrogen in the reactor via the water-gas shift reaction. Water and hydrogen are needed to react with precipitated Rh(III) and inactive [RhI4(CO)2] to regenerate the active Rh(I) catalyst. The large amount of water increases the amount of hydrogen iodide, which is highly corrosive and leads to engineering problems.
Millennium Petrochemical Company developed a new rhodium carbonylation catalyst system. The catalyst system uses a pentavalent Group VA oxide such as triphenylphosphine oxide as a catalyst stabilizer. The catalyst system not only reduces the amount of water needed but also increases the carbonylation rate and acetic acid yield. See U.S. Pat. No. 5,817,869.
One challenge still facing the industry is that lowering water concentration in the methanol carbonylation results in increased aldehyde formation. Methods for reducing aldehyde concentration in acetic acid are known. For instance, U.S. Pat. No. 7,208,625 teaches reducing aldehyde and other permanganate-reducing impurities from acetic acid by contacting acetic acid with peracetic acid and air. Co-pending application Ser. Nos. 11/496,900 and 11/508,109 teaches removing aldehyde impurities from acetic acid by reacting the aldehydes with hydroxyl compounds to form acetals which are then removed from acetic acid by distillation. Co-pending application Ser. No. 11/810,167 teaches extracting the aldehyde impurities with a polyol from a methyl iodide solution in the acetic acid production process.
Methyl acetate is commonly used in methanol carbonylation. One source of methyl acetate is the byproducts of polyvinyl acetate hydrolysis or methanolysis. These byproducts are often supplied as mixtures of methyl acetate and methanol and they contain various amounts of aldehyde impurities. It is important to remove aldehyde impurities from the methyl acetate supply because these impurities complicate the methanol carbonylation and increase the impurity level in the acetic acid product. However, some aldehyde impurities, such as acetaldehyde, are not easy to separate from methyl acetate because of their close boiling points. Thus, new methods for removing aldehyde impurities from methyl acetate are needed.